Avoid vibration issues due to engine power limitation
Engine power limitation (EPL) is one possible solution to fulfil the requirements of the IMO’s Energy Efficiency Existing Ship Index (EEXI). EPL is a system to improve a ship’s energy efficiency by limiting its engine power with the aid of a fuel index limiter, a simple device on the ship’s engine control system. An alternative EPL solution is the deactivation of one or more cylinders. Not requiring the retrofit of a complicated system, EPL can be utilized as one of the effective measures to improve energy efficiency of existing ships in the short-term.
What is the concern?
In most cases, limiting the power range goes along with a limitation of the operational revolution range. The new operational revolution rate may cause resonance conditions, leading to increased structural or machinery vibrations. Higher vibration levels may result in crew discomfort, structural damages and machinery failure. If power limitation is realized by deactivating cylinders, this will completely change the engine’s dynamics, requiring a new assessment of shaft torsional vibrations as well as of shipboard vibration due to changed firing frequency as well as modified excitation forces and moments.
What is DNV GL’s solution?
The risk of resonance can be assessed based on documents that are typically already available from the design phase of the vessel:
- Vibration measurements conducted during the sea trial
- Torsional and axial shaft vibration calculations
- Numerical global and local vibration analyses
Within the EEXI Vibration Pre-Check, DNV GL’s vibration experts screen the documents available with respect to completeness and informative value. Based on the outcome of this screening, subsequently either the risk of resonances will be assessed or recommendations will be given on how to overcome the information gap. If a risk of resonances or of increased vibrations is identified, practicable and cost-effective solutions are proposed and discussed.